1. Technical Field
This invention relates to an ATM (Asynchronous Transfer Mode) switch, and particularly to a congestion control method and a priority control method during reception of the connection setup requests in an SVC (Switched Virtual Channel) service to establish the connection among the ATM switches on demand.
2. Description of Related Art
The ATM switch is equipped with an ATM interface through which a plurality of ATM devices (ATM apparatus) such as ATM terminals, ATM switches, ATM concentrators, ATM routers etc. are respectively connected. An ATM network comprises a plurality of these ATM devices.
When ATM devices are required to be connected in the SVC service of an ATM switch, the ATM device sends a signaling message requesting a SETUP of the connection to the ATM switch to negotiate a destination, a frequency band and so on, so as to establish the connection between the ATM device and the ATM switch.
When a plurality of the ATM switches exist between a calling ATM device (originator) and the called ATM device (destination), the above negotiation process is executed for every two adjacent ATM devices. To process the signaling messages transmitted from a plurality of the ATM devices, the ATM switch is equipped with a buffer for receiving the signaling messages. The ATM switch stores the signaling messages from the ATM devices in the buffer temporarily and then extracts the signaling message from the buffer to execute signaling processes such as establishment of the connection.
Congestion may occur when a large number of signaling messages in excess of the signaling process capability of the ATM switch are received at the ATM switch because of, for example, booting of all ATM devices, requesting of a short period connection setup by a high performance ATM device, malfunction/failure of ATM devices, and so on. Once the buffer which receives and stores the signaling messages sent from ATM devices has overflowed, all the signaling messages received after the overflow are discarded at cell level or packet level until a condition of the buffer is restored in a conventional ATM switch.
The possibility of overflow may be reduced in the conventional device if a capacity of the buffer for receiving the signaling message is made larger. However, the signaling message may be stored in the buffer for a longer period of time if a larger buffer capacity is implemented. The ATM device which sent the signaling message to request the connection setup usually monitors a time from transmission of the signaling message to receipt of the response message. Thus, if the signaling message is stored for a longer period of time, it is possible for the originator of the signaling message to re-send another signaling message because of overtime delay. Particularly, once congestion has occurred, discarding of the connection setup requests from all ATM devices connected to the ATM switch may cause re-sending of the connection setup requests from these ATM devices. The signaling process capability may also be increased. However, the signaling process capability still has its own limit and may not resolve the congestion problem.
Furthermore, in the ATM switch, each of the signaling messages from ATM devices is processed according to its receiving sequence. The signaling message is provided with information elements defining a type of the connection, such as a type of service to be used, cell speed, etc. Since some types of signaling messages require the use of a fixed frequency band, the signaling message coming after such a signaling message may be rejected because of a lack of available frequency band.
Among ATM devices, there are important devices such as ATM ARP (Address Resolution Protocol) servers, LAN emulation servers, which make interconnections with a conventional LAN on an ATM-LAN system, and devices such as file serves, and printer servers, which are frequently accessed by users. In some cases, it is desirable to process the signaling messages from these ATM devices giving them preference over the signaling messages from ATM devices of general users.
When the connection is established via a plurality of ATM switches, it means that a frequency band to be used has been secured in every one of the ATM switches on the route. If the latter ATM device rejects the connection setup request because of a lack of frequency band or the like, these secured frequency bands in the rest of the ATM switches on the route will be wasted. It also possible that the signaling messages may not be received by the ATM devices on the route.